In this paper, we give the far-field diffraction theory for calculating the divergence by using the superposition of Gaussian beams instead of the aperture function and obtain the explicit express for calculating the beam divergence. We analyze in detail the influence of the diffraction adding defocus aberrations upon the transmitter's divergence and get the simulated result. These researches are useful to system design and give us some suggestions of avoiding the influence of the aberrations and the diffractions on the transmitter.
In this paper, a prototype of intersatellite laser communication terminal for a principle demonstration is reported, and the corresponding ground support equipments are described, too. The terminal has two main subsystems, the first is one for the laser communication and the second for the pointing, acquisition and tracking. Laser diodes are used for the communication link, and with the average laser power of more than 200mW and the data-rate up to 600Mbps. The PAT unit consists of a fine pointing mechanism and a coarse pointing assemble, which reaches a tracking accuracy of ~5μrad. The on-ground test equipments are included in a communication test bed with a long-distance beam propagation simulator, a PAT test bed with an optical satellite trajectory simulator, and a wavefront test bed with deferent lateral-shearing interferometers.
In this paper, a subsystem of coupling semiconductor laser diode to the single-mode fiber and producing the diffraction-limited beam is described in detail, and beam wavefront is measured by a double shearing-interferometer experimentally. As a result, the divergence of the collimated beam is only 65.84μrad, which indicates that the transmitted beam achieves the diffraction-limited, and this optical subsystem is coincident very well with our design.
Grating spacing has great influence on grating formation via electron transport in the conduction band during holographic recording. Grating spacing dependence of two center holographic recording is investigated by writing nonvolatile holograms in doubly doped LiNbO3 crystals. Several types of electron characteristic transport lengths are defined. The effects of electron characteristic transport lengths and oxidation-reduction state on the amplitude of nonvolatile holographic grating are numerically discussed within the range of grating spacing from 0.14μm to 10μm. It is found that the bulk photovoltaic effect is the dominant electron transport mechanism of doubly doped LiNbO3 crystals, while the diffusion effect does have appreciable effect on the grating formation when grating spacing is smaller than 1μm. Theoretical analysis and experimental results confirm that the logarithm of the amplitude of nonvolatile holographic grating Log (δn) increases with increasing grating spacing Λ within region of small grating spacing (Λsmaller than 1μm) and further increasing of grating spacing won’t influence the values of Log (δn) greatly.